25 May 2011

Do bacteria create hail stones?

Agence France-Presse
A high concentration of bacteria have been discovered in the centre of hailstones, suggesting that airborne microorganisms may be responsible for that and other weather events.
hail stones

Credit: iStockPhoto

WASHINGTON: A high concentration of bacteria have been discovered in the centre of hailstones, suggesting that airborne microorganisms may be responsible for that and other weather events.

Scientists in the U.S. analysed hailstones over 5 cm in diameter that were collected after a storm in June 2010. The large hailstones were seperated into four layers and the meltwater from each layer was analysed. The number of culturable bacteria was found to be highest in the inner cores of the hailstone.

“Bacteria have been found within the embryo, the first part of a hailstone to develop. The embryo is a snapshot of what was involved with the event that initiated growth of the hailstone,” said Alexander Michaud of Montana State University in Bozeman, who presented the research at the 11th General Meeting of the American Society for Microbiology in New Orleans.

“The embryo is a snapshot of what was involved with the event that initiated growth of the hailstone,” said Michaud, a lead researcher in the field of bioprecipitation, the study of how bacteria may cause rain, snow and hail.

Cracking open the ice nucleus

Clouds can only make ice, from which snow can fall, if a particle is present for the ice crystals to grow around, known as an ice nucleus.

“In order for precipitation to occur, a nucleating particle must be present to allow for aggregation of water molecules,” said Michaud. “There is growing evidence that these nuclei can be bacteria or other biological particles.”

Michaud’s research is part of a growing field of study focusing on bioprecipitation, a concept where bacteria may initiate rainfall and other forms of precipitation including snow and hail. The formation of ice in clouds, which is necessary for snow and most rainfall events, requires ice nuclei (IN), particles that the ice crystals can grow around.

“Aerosols in clouds play key roles in the processes leading to precipitation due to their ability to serve as sites for ice nucleation. At temperatures warmer than -40 degrees Celsius ice formation is not spontaneous and requires an IN,” said Brent Christner of Louisiana State University, who also presented at the meeting.

Influencing the size of crystals

A plant pathogen known as Psuedomonas syringae is commonly at the root of precipitation events, because its outer surface is so efficient at collecting water molecules around it, according to Christner.

“Ice nucleating strains of P. syringae possess a gene that encodes a protein in their outer membrane that binds water molecules in an ordered arrangement,” he said. This provides “a very efficient nucleating template that enhances ice crystal formation”.

Aerosol-cloud simulation models imply that high concentrations of biological IN may influence the average concentration and size of ice crystals in clouds, horizontal cloud coverage in the free troposphere, precipitation levels at the ground and even insulation of the earth from solar radiation.

“Evidence for the distribution of biological IN in the atmosphere coupled with the warm temperatures at which they function as IN has implied that biological IN may play a role in the Earth’s hydrological cycle and radiative balance,” said Christner.

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